Field of the Invention
Powder coatings are solid compositions which are generally applied by an electrostatic spray process in which the powder coating particles are electrostatically charged by the spray gun and the substrate is earthed. Alternative application methods include fluidised-bed and electrostatic fluidised-bed processes. After application, the powder is heated to melt and fuse the particles and to cure the coating.
The compositions generally comprise a solid film-forming resin, usually with one or more colouring agents such as pigments, and optionally they also contain one or more performance additives. They are usually thermosetting, incorporating, for example, a film-forming polymer and a corresponding crosslinking agent (which may itself be another film-forming polymer). Generally, the resins have a Tg, softening point or melting point above 30° C.
The compositions are generally prepared by mixing ingredients, e.g. in an extruder, at a temperature above the softening temperature of the resin but below the curing temperature. The composition is then cooled to solidify it and is subsequently pulverised. The particle size distribution required for most commercial electrostatic spray apparatus is up to a maximum of 120 microns, with a mean particle size within the range of 15 to 75 microns, preferably 25 to 50 microns, more especially 20 to 45 microns.
The present invention relates to a method for applying a powder coating to a substrate, more in particular to a process for applying at least two powder coating layers to a substrate without any substantial curing of the first layer prior to the application of the second or further layers. This process is sometimes referred to as a dry-on-dry application process.
Prior Art
In WO 97/05965 a method is disclosed for simulating wood or marble in a finish by coating metal surfaces with a first layer of a colored powder coating layer, heating this layer to get a partial cure of this first layer (sometimes referred to as green cure), and thereafter applying a second colored powder coating layer, and subsequent heating of both layers to obtain a full cure of both layers.
In EP 1547698 a method is disclosed which is similar to the process in WO 97/05965, albeit that in the process of EP 1547698 the heating step after the application of the first powder coating layer is absent.
In WO 2008/088605 a method is disclosed for painting a substrate wherein in a first step a powder primer is applied to the substrate, in a next step a powder basecoat comprising a flake additive is applied onto the primer, the powder primer and powder basecoat are simultaneously cured and thereafter a topcoat is applied onto the powder basecoat and in a last step this topcoat is cured. It was found that this method only works at very specific conditions and control of certain parameters, such as layer thickness, charging of the powder particles, and settings of the application equipment.
In EP994141 a method is disclosed which is similar to the process in WO 2008/088605. In some of the examples carbon black is added in small amount (<1 wt. %) to the primer coating layer. However, it was found that in a process where the powder basecoat and powder topcoat are simultaneously cured, this only works at very specific conditions and control of certain parameters, such as layer thickness, charging of the powder particles, and settings of the application equipment.
In US 20060110601 a process for the application of a flouropolymer primer and overcoat is disclosed that uses a process similar to the process in WO 2008/088605. It is said that metal flake and carbon black can be added as filler. It is advised that the powder primer coating should contain 10 to 20 wt. % of such filler.
In EP 2060328 a method for forming a composite powder coating is disclosed wherein multiple layers of a powder coating are deposited on a substrate, wherein adjacent layers are formed of different types of powder coating compositions and wherein the multiple layers of the powder coating composition are cured in a single thermal step.
In WO 2005/018832 a method for coating substrates is disclosed wherein an image coat is applied over a background coating. Both image coating and background coating can be powder coatings. It is not necessary to partially cure the background coating before the image coat is applied. In this process the polarity of the background/base coat and the image coat must be the same.
In US 2004/0159282 a respray or repair coating method using powder coatings is disclosed where the respray or repair coating may be performed before or after the cure of the initial layer. The initial coating layer and the repair/respray coating layer should have the same electrostatic polarity.
So far, there has been little commercial success for systems based on any of the above processes for the dry-on-dry application of at least two powder coating layers. Main reasons for this are surface defects in the top powder layer which, when cured lead to an unsatisfactory appearance with evidence of mixing of the two layers. These surface defects can be masked by using a matt or dull colored coating for the top powder layer. However, the surface defects are clearly visible when a high gloss topcoat is used.